A galaxy with a stellar mass of M ≥ 1011Msun and an
→ effective radius of Re ≤ 1.5 kpc.
Many studies have shown that massive galaxies with low
→ star formation rates
were remarkably compact at a → redshift of
z≥ 2. At fixed stellar mass of
Mstars ≅ 1011Msun,
quiescent galaxies are a factor of ~ 4 smaller at z = 2 than at z = 0.
As the stellar mass of the galaxies
also evolves, the inferred size growth of individual galaxies is even
larger. It is unlikely
that all massive galaxies in the present-day Universe had a compact
progenitor. However, the
vast majority of CMGs that are observed at
z = 2 ended up in the center of a much larger galaxy today. Their size growth
after z = 2 is
probably dominated by minor → mergers.
Such mergers are expected because
other mechanisms cannot easily produce the observed scaling between
size growth and mass growth
(P. G. van Dokkum1 et al., 2015, ApJ 813, 23).

Massive star formation outside → OB associations.
Recent observational findings suggest that → massive star
formation is a collective process. In other words, massive stars form in
→ cluster environments
and the mass of the most massive star in a cluster is correlated with the mass of the
cluster itself. Nevertheless, other observational results give grounds for supposing that
massive stars do not necessarily form in clusters but that they can be formed
as isolated stars or in very small groups. According to statistical studies
nearly 95% of Galactic → O star
population is located in clusters or OB associations.
This means that a small percentage, about 5%, of high mass stars may
form in isolation. Isolation is meant not traceable to an origin
in an OB association. This definition therefore excludes
→ runaway massive stars,
which are thought to result from either dynamical interaction in massive dense clusters,
or via a kick from a → supernova explosion in a
→ binary system. Alternatively, isolated massive star has been
defined as follows: An O-type star belonging to a cluster whose total mass is
< 100 Msun and moreover is devoid of → B stars
(Selier et al. 2011, A&A 529, A40 and references therein).

A black hole with a mass between millions and billions of solar masses
residing in galactic nuclei. The mass of this type of black holes represents about 0.2%
of the bulge mass. When matter is swallowed by the black hole, this gives rise to the
tremendous energetic phenomena observed in quasars and active galactic nuclei.

A → black hole of tremendous mass, equivalent to those of
millions or even billions of stars, which is believed to exist and occupy the centers
of many galaxies. The supermassive black hole residing in the center of our
→ Milky Way Galaxy is
the object → Sgr A* with a mass of 4 x 106→ solar masses within a radius of 100
→ astronomical units.

A star with an initial mass over about 120 solar masses.
The existence of such stars is the present Universe is not confirmed.
Such stars were proposed as an explanation for very bright O type stars in the Large Magellanic
Cloud, but these are now known to be clusters of ordinary O stars.
→ very massive star;
→ massive star.